What is the difference between gastrulation and organogenesis?

My notes from class aren't very clear and from what I've found online, they seem to sort of blend into each other.... Can anyone give me a clear definition, please?
Update: @Shelby: How insightful!
Update 2: @LaRa and Origin in disguise: Okay, that makes sense. The part that was getting me was those three layers. Thanks!
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